Thermal printing head

ABSTRACT

A thermal printing head comprising a semiconductor body and a heat-generating resistive layer formed by diffusing an impurity into a surface portion of the semiconductor body by any suitable known method. In the thermal printing head, silicon having a low resistivity is employed to form the semiconductor body and an impurity having a conductivity opposite to that of the semiconductor body is employed to form the heat-generating resistive layer so as to completely concentrate current in the resistive layer and attain remarkable improvements in the heat generating characteristic of the thermal printing head.

United States Patent V [1 1 ()tani et al.

11m. 3,814,897 1 June 4, 1974 THERMAL PRINTING HEAD [75] Inventors:lliroshi Otani, Shijonawate; Noboru Yukami, Hirakata, both of Japan [73]Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan 221 Filed: Feb. 29, 1972 21 Appl.No.:230,332

[30] Foreign Application Priority Data Mar. 4, 1971 Japan ..L 46-11675[52] 11.5. C1 219/216, 219/543, 317/234 Q, 346/76 R [51] Int. Cl. H051)1/00 [58] Field of Search 219/216, 543; 346/76 R; 1 178/30; 29/569, 590;317/234 Q [56] References Cited UNITED STATES PATENTS 1161.457 12/1964Schroeder et a1 346/76 R 9/1971 Davis et a1. 219/216 X Mosher [57]ABSTRACT A thermal printing head comprising a semiconductor body and aheat-generating resistive layer formed by diffusing an impurity into asurface portion of the semiconductor body by any suitable known method.In the thermal printing head, silicon having a low resistivity isemployed to form the semiconductor body and an impurity having aconductivity opposite to that of the semiconductor body is employed toform the heat-generating resistive layer so as to completelyconcentr'ate current in the resistive layer and attain remarkableimprovements in the heat generating characteristic of the thermalprinting head.

5 Claims, 8 Drawing Figures PATENTEnJun 4mm v 381 2.89?

. saw Mr 4 I 1' i THERMAL PRINTING HEAD This invention relates to athermal printing head for use in a device which is adapted to printinformation on a heat-sensitive recording medium by means of thermalenergy.

Various types of heat-sensitive sheets of paper which produce colors inresponse to the application of thermal energy are now available on themarket. Thermal printing heads of the kind in which head is generated bythe concentrated flow of current through a resistor portion have beendeveloped and are now available on the market. The thermal printingheads of this kind can be broadly classified into two types. In one typeof thermal printing heads, a plurality of resistor portions are providedon-one end surface of a body of material having a high resistivity as anintegral part of the body and current is passed through the resistorbank for generating heat in the resistor bank, while in the other type,a semiconductor such as silicon is used as a heatgenerating resistor andcurrent is supplied to thisresistor through a supporting membersupporting the resistor so as to cause generation of heatin theresistor.

In the thermal printing head of the former type, the resistance of thebody must be sufficiently higher than that of the heat-generatingresistor portions. Practically, an electrical insulator such as glass orceramics is used as the material for the body, and a compound such astin oxide is used to form the resistor portions. In the thermal printinghead of the latter type in which the body having a high resistivity isformed from a semiconductor such as silicon, the impurity concentrationin a surface portion of the semiconductor body is selectively increasedby a known method of selective diffusion or the like so as to reduce theresistance of the surface portion to a value extremely lower than thatof the semiconductor body. The thermal printing head of the latter typeis appreciated in that the greater part of current can be concentratedin the surface portion doped with the impurity thereby generatingsubstantial heat in such portion.

It is a primary object of the present invention to provide a thermalprinting head of novel and improved construction showing an improvedheat generating characteristic over conventional ones.

The present invention is featured by the fact that, in a thermalprinting head comprising a semiconductor body and a heat-generatingresistive layerformed by diffusing an impurity into a surface portion ofthe semiconductor body by any suitable known method, silicon having alow resistivity is employed to form the semiconductor body and animpurity having a conductivity opposite to that of the semiconductorbody is employed to form the heat-generatin g resistive layer so as tocompletely concentrate current in the resistive layer and attainremarkable improvements in the heat generating characteristic of thethermal printing head.

The above and other objects, features and advan-' tages of the presentinvention will be apparent from the following detailed description of apreferred embodiment thereof taken in conjunction with the accompanyingdrawings, in which: I

FIGS. 1A and 1B are a perspective view and a longitudinal sectionrespectively of a thermal printing head according to the presentinvention;

FIG. 2 is a perspective view of an array of thermal printing head chipsmade in accordance with the present invention;

FIGS. 3A to 3D show successive steps for the manufacture of the thermalprinting head according to the present invention; and

FIG. 4 is a graph showing variations in the resistivity of asemiconductor relative to temperatures.

Referring to FIGS. IA and IB, a thermal printing head according to thepresent invention comprises a semiconductor body 11 of p-type or n-typesilicon having a low resistivity. A resistive layer I2 is formed in asurface portion including one end of the semiconductor body 11 by aknown diffusion method and serves as a heat generating layer. Thisresistive layer 12 is of the n-type and p-type when the semiconductorbody 11 is of the p-type and n-type respectivelythereby forming a pnjunction 15 therebetween. A pair of electrodes 13 and 13' are in ohmiccontact with the resistive layer I2 so that, when a voltage is appliedacross these electrodes 13 and 13', concentrated flow of current passesthrough the resistive layer 12 thereby generating substantial heat inthe resistive layer 12. A layer 14 of electrical insulator such as asilicon oxide film is formed on the semiconductor body 11 so as toprevent the electrodes 1 3 and 13 from contacting the surface portionsof the semiconductor body 11 which are not doped with the impurity.Colors are produced on a heatsensitive sheet 16 at portions which areengaged by the hot head. It will be understood that such a thermalprinting head can be constructed to have any desired size, and aplurality of thermal printing head chips of any suitable shape can alsobe arranged in an array as seen in FIG. 2. Referring to FIG. 2, aplurality of thermal printing head chips 18 are arranged in a line withan electrical insulator 17 interposed therebetween.

A method of making the thermal printing head of above constructionwillnow be described with reference to FIGS. 3A to 3D. i

A semiconductor body 21 of silicon having a low resistivity is preparedat first as shown in FIG. 3A. A perspective view and a side elevation ofthis semiconductorbody 21 are shown on the left-hand side and righthandside respectively of FIG. 3A. This semiconductor body 21 is pre-formedto have a height h and a thickness w, which conform to the predeterminedheight and thickness of a thermal printing head to be finally produced.Then, as shown in FIG. 3B, substantial portions of the opposite sidesurfaces of the semiconductor body 21 are covered with an oxide f lm 23except the end surface 22 and those surface portions lying within asuitable distance I measured from the end edges of the end surface 22.This can be realized by, for example, thermally oxidizing the entiresurfaces of the semiconductor body 211 and then removing the oxide filmlying within the above range by photo-etching. After the oxide film hasbeen partly removed, an impurity is diffused into the exposed ornon-oxidized surface portions of the semiconductor body 21 as shown inFIG. 3C for forming a diffused layer 24 in the surface portionsincluding the end surface 22 and the side surface portions covering thedistance I from the end edges of the end surface 22 of the semiconductorbody 21. This diffused layer 24 acts as a resistive layer. A heatgenerating portion for applying thermal energy to a heatsensitiverecording medium is preferably provided on a limited area, and in thisrespect, the provision of the 3 1 heat generating portion solely on theend surface 22 is essentially required as will be apparent from FlG. 1B.The area of the surface portions covering the length 1 from the endedges of the end surface 22 is preferably as small as possibleconsidering the fact that heat is wastefully lost in such portions.Therefore, the area of these surface portions should be limited to aminimum which is enough for electrical connection with the electrodes.A' known method of vapor phase diffusion may be employed for thediffusion of the impurity. An impurity such as phosphorus belonging tothe group V is diffused when the semiconductor body of low resistivityis of the p-type, while an impurity such as boron belonging to the grouplll is diffused when the semiconductor body is of the n-type. lt isneedless to say that the resistance of the resistive layer can becontrolled as desired by suitably diffusing the impurity. Then, as shownin FIG. 3D, a metal such as aluminum or nickel is deposited to provide apair of electrodes 25 and 25' so as to extend over the oxide film 23 andover substantial portions of the diffused layer 24 on the side surfacesofthe semiconductor body 21. A method of vac-' uum evaporation may beemployed for the deposition of the electrodes 25 and 25'. The vacuumevaporation may be carried out while masking the end surface 22 so thatthe metal may not be deposited on the end surface 2 2, or afterevaporating the metal on the entire surfaces, predetermined portions maybe removed by photo-etching I and the electrode portions may be shapedto the desired form. Subsequently, a cutting means such as a diamondcutter or wire saw may be used to cut the head into a size which has awidth slightly larger than a predetermined width w After lapping theworked surfaces of the head, chemical etching is applied to the head forreshaping the pn junction 26, which has been impaired during the cuttingoperation, and adjusting the size of the head so that it has thepredetermined width W2 v The thermal printing head may be required toproduce a temperature higher than, for example, 400 C depending on thekind of the heat-sensitive recording medium. In the case of conventionalthermal printing heads of the type in which silicon having a highresistivity is used as a semiconductor body and an impurity doped layerhaving a low resistivity is formed in the surface portion of thesemiconductor body for obtaining a heat generating layer, the heatgenerating characteristic of the head is necessarily deteriorated when ahigh temperature is produced due to the heat generated in the heatgenerating layer. In the case of the head according to the presentinvention, however, a high temperature can be satisfactorily producedwithout deteriorating the heat generating characteristic thereof for thereasons described below.

FIG. 4 shows the temperature characteristic of the lustrated by way ofexample, and the critical tempera ture becomes remarkably lower at ahigher resistivity. An increase in the temperature of the limitedportion in contact with a heat-sensitive recording medium is enough fora thermal printing head to act as a heat generator, and the thinner thethickness of the heat generating portion, the thermal energy produced inthis portion is more effectively utilized. ln the conventional thermalprinting head of the type in which the resistance of the shallow regionalong the surfaces of a semiconductor body having a high resistivity isreduced by the diffusion of an impurity thereby establishing asufficiently large difference between the resistance of the dopedsurface portion and that of the interior portion of the semiconductorbody and concentrated flow of current is passed through the dopedsurface portion for generating a large amount of heat in the dopedsurface portion, it is desirable to pass the current through the portionwhich is as near the surface of the semiconductor body as possible, andthus the silicon crystal forming the semiconductor body should have aresistivity as high as possible. In the silicon crystal having such ahigh resistivity, however, a transition to the intrinsic region takesplace readily with an increase in the temperature, and at a temperaturehigher than the critical point, the resistance of the semiconductor bodyis abruptly reduced with the result that current flows also through theinterior of the semiconductor body and considerable heat is generated inthe interior of the semiconductor body. Consequently, a large amount ofresistivity of silicon. It will be seen from FIG. 4 that the resistivityof silicon increases with the increase in the temperature within atemperature range higher than room temperature and starts to decreaseabruptly beyonda certain critical point. The region in which such abruptdecrease in the resistivity occurs is called an intrinsic region. Thecritical temperature at which a transition-to this intrinsic regiontakes place varies depending on the resistivity, and the higher theresistivity, the lower the critical temperature. In FIG. 4, the criticaltemperatures of silicon crystals having respective resistivities'of 0.70cm, 1.3 Qcm, 4 flcm and 10 Om are ilheat is generated not only in thesurface portion to be brought into contact with a heat-sensitiverecording medium but also in the interior of the semiconductor body, andthe heat generating efficiency of the thermal printing head is extremelylowered.

On the other hand, according to the present invention, a resistive layeris formed in the surface portion of a semiconductor body of silicon bydiffusing an impurity having a conductivity opposite to that of thesemiconductor body so that the path of current is lim ited to one sideof the pn junction and substantially all the current flows through theresistive layer. In this case, therefore, the semiconductor need nothave a high resistivity. A large amount of heat is generated in theinterior of the semiconductor body as is the case with the conventionalhead only when breakdown of the pn junction occurs to allow for flow ofcurrent into the interior of the semiconductor body. However, it is saidthat the so-called secondary breakdown of a pn junction occurs only whena transition to the intrinsic region takes place in a semiconductorportion having a high resistivity. lnasmch as the present inventionemploys a semiconductor body of silicon having a low resistivity, thepresent invention is advantageous over the conventional thermal printingheads in that the path of current is limited to the resistive surfacelayer up to elevated temperatures and the head is free from anyundesirable reduction in the heat generating efficiency up to suchelevated temperatures.

What is claimed is:

l. A thermal printing head which generates heat for thermally recordinginformation on a heat sensitive recording medium, comprising:

a body of low resistivity semiconductor material having a criticaltemperature at which a transition to the intrinsic region takes placewhich is higher than a color producing temperature of said heatsensitive recording medium;

a layer of opposite conductivity type from said semiconductor bodyformed on the surface of said body facing said heat sensitive recordingmedium, said layer extending over and part way down opposite sides ofsaid body to form a semiconductor junction with said body;

an electrically insulating layer covering at least that portion of eachside surface of said semiconductor body immediately adjacent to andextending away from said opposite conductivity type layer; and

an electrode formed on each said insulating layer and it extending ontosaid opposite conductivity type layer in contact therewith, a heatgenerating conductivity path being formed through said oppositeconductivity type layer between each said electrode.

2. A thermal printing head according to claim 1,

wherein each said electrode is disposed in ohmic contact with saidopposite conductivity type layer.

' 3. A thermal printing head according to claim 2, wherein said oppositeconductivity type layer comprises a doped impurity layer diffused intosaid semiconductor body.

4. A thermal printing head according to claim 3, further comprising aplurality of said opposite conductivity type layers formed on saidsurface of said semiconductor body and a plurality of electrodes formedon said insulating layer and in contact with corresponding ones of saidplurality of opposite conductivity type layers, each oppositeconductivity type layer and its corresponding electrodes being separatedfrom each other opposite conductivity layer and associated electrodes.

5. A printing head according to claim 4, wherein said body ofsemiconductive material is composed of one of n-type and p-typematerials.

1. A thermal printing head which generates heat for thermally recordinginformation on a heat sensitive recording medium, comprising: a body oflow resistivity semiconductor material having a critical temperature atwhich a transition to the intrinsic region takes place which is higherthan a color producing temperature of said heat sensitive recordingmedium; a layer of opposite conductivity type from said semiconductorbody formed on the surface of said body facing said heat sensitiverecording medium, said layer extending over and part way down oppositesides of said body to form a semiconductor junction with said body; anelectrically insulating layer covering at least that portion of eachside surface of said semiconductor body immediately adjacent to andextending away from said opposite conductivity type layer; and anelectrode formed on each said insulating layer and extending onto saidopposite conductivity type layer in contact therewith, a heat generatingconductivity path being formed through said opposite conductivity typelayer between each said electrode.
 2. A thermal printing head accordingto clAim 1, wherein each said electrode is disposed in ohmic contactwith said opposite conductivity type layer.
 3. A thermal printing headaccording to claim 2, wherein said opposite conductivity type layercomprises a doped impurity layer diffused into said semiconductor body.4. A thermal printing head according to claim 3, further comprising aplurality of said opposite conductivity type layers formed on saidsurface of said semiconductor body and a plurality of electrodes formedon said insulating layer and in contact with corresponding ones of saidplurality of opposite conductivity type layers, each oppositeconductivity type layer and its corresponding electrodes being separatedfrom each other opposite conductivity layer and associated electrodes.5. A printing head according to claim 4, wherein said body ofsemiconductive material is composed of one of n-type and p-typematerials.